Accessing Apex rest class with OAuth 2.0 with Named Credential authentication mechanism

I have been trying to implement Oauth 2.0 in my org and my requirement is to use Named credentials only when making a apex callout. I am using Named credentials, Oauth, Connected Apps and auth. provider to establish a connection with an external Salesforce org. The external org has exposed an apex class as REST service that I am trying to access using my org. I have followed the steps given in this link to setup everything :